This invention relates to the field of current mode active filters and more particularly to active filters utilizing a minimum of components in an IC implementation while providing a relatively high Q and improved dynamic range.
A second order active filter is described in U.S. Pat. No. 4,340,868, issued July 20, 1982, and entitled "Current Mode Biquadratic Active Filter". The active filter in this patent has been completely implemented on an integrated circuit which operates from a supply voltage of 1 volt. The maximum filter Q obtainable from this circuit is limited by finite transistor beta and the maximum capacitor ratio realizable on an IC. The dynamic range of the input and output signals is limited by the non-linear diode voltage-current characteristic at one or both of the main circuit nodes. Input signals with an amplitude greater than several millivolts peak-to-peak can cause amplitude and phase distortion at the filter output. If the input signal level is kept small to eliminate dynamic range problems, then filter performance can be degraded due to poor signal-to-noise ratio.
In some low-frequency filter designs the required filter capacitor sizes are too large to implement on an IC. In this case external discrete components are used and the number of external parts and IC package pinouts required becomes important in order to minimize product size and cost. In other filter designs, additional external components are needed in order to have the capability to externally program filter frequency.